1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electro-photographic process, and more particularly to an image forming apparatus that enables shortening of the warming-up length of time for a fixing unit.
2. Description of the Prior Arts
In an image forming apparatus such as a digital type copying machine and a digital type composite machine wherein functions of a printer, facsimile, etc. are integrated in that copying machine, ordinarily, there is used a printer part that uses an electro-photographic process. In such image forming apparatus using the printer part, in many cases, a relatively long length of time is needed to execute warming-up after making the power ON. From a viewpoint of the convenience in use as well as energy saving, shortening of that length of time has hitherto been demanded and various kinds of techniques for meeting such demand have hitherto been developed.
For example, there is a technique that calculates from the size and the number of recording mediums a total area of the images that will be formed determines the temperature of the fixing unit that is needed to perform fixing with respect to that total area of image, and withdraws from heating to an extent that is greater than necessary, thereby achieving shortening the warming-up length of time (Japanese Patent Application Laid-Open (JP-A). No. 10-39674). Also, there is a technique that determines consuming amount of toner information from the thickness of an original document and the number of black dots in each original document, determines a temperature at which the toner is fixable according to that information, and, at the point of time when the temperature of the fixing machine has been raised up to this temperature, determines that warming-up is complete (JP-A No. 11-125987).
On the other hand, there is a technique that, in order to shorten the warming-up length of time, performs reduction in heat capacity (for example reduction in wall thickness) of the heating member for the fixing unit and that, as this heating part, uses an element that directly generates heat through the use of eddy current, thereby aiming to enhance the efficiency of heating. (JP-A No. 7-295414)
By the way, in the case of an electro-photographic type image forming apparatus, after an allowing-to-stand period of time has been done, as at the earliest time in the morning, for a long period of time without image formation processing being executed, the electrical chargeability of the developer and the photo-sensitivity characteristic of the photosensitive body become unstable. As a result of this, it was very likely that inconveniences in the quality of image such as excess in the image concentration and deficiency in the image concentration or collapse and getting out of sequence in the gradation would take place. These inconveniences in the quality of image are eliminated by performing the preliminary rotating operation for the photosensitive body and the preliminary agitating operation for the developer, by, after performing those preliminary operations, forming more than one visible image of a test image on the photosensitive body while changing the developing conditions, by detecting the concentrations of them, and by thereafter correcting the developing conditions so that an appropriate value of concentration is obtained.
Thereupon, there has been proposed an image forming apparatus that is configured so that the above-described preliminary operation and image correction operation are performed for several minutes that are required for the fixing warming-up operation (JP-A No. 11-38700). FIG. 1 illustrates the flow of operation for warming-up where image correction operation is executed during the fixing warming-up period of time. When the power for the apparatus is made ON (step S801), the fixing warming-up operation (step S802), preliminary rotation operation of the photosensitive body (step S803), and preliminary rotating operation of the developing unit (step S804) are executed in parallel. When the fixing unit has been heated up to a prescribed temperature at which warming-up is complete (for example 180° C.), the fixing warming-up operation is terminated (step S805; Y). Thereby, fixing heater is switched OFF (step S806).
The preliminary rotating operation for the photosensitive drum and that for the developing unit are complete (step S808 and step S810) after lapse of 120 seconds (step S807; Y and step S809; Y). Thereafter, an image correction process is performed to examine the developing conditions on which an appropriate concentration is obtained (steps S811 and S812) by forming more than one image of the test image on the photosensitive body by changing the developing conditions. When both of the fixing warming-up operation and image correction process steps have terminated and the whole warming-up operations have thereby completed (step S813; Y and step S814), a message such as “ready to copy” is displayed on the operation/display panel (step S815). As a result, the apparatus is brought to a state ready for use.
FIG. 2 illustrates the operational timings of respective parts of the image forming apparatus in a case where the image correction operation has terminated before completion of the fixing warming-up. The power is made ON at time T1 and the fixing warming-up starts at time T2 . At time T3, the preliminary rotating operations for the photosensitive body and developing unit starts. These preliminary rotating operations are terminated at T4 and, at time T5, the photosensitive body and developing unit rotate for formation of test images. From time T6 to time T7, image correction operation for forming visible images of test images while changing the developing conditions is executed. Upon cease of the rotations of the photosensitive body and developing at time T8, image correction operation is complete. Thereafter, at time T9, warming up of the fixing heater is terminated and, at this point of time, warming up of the apparatus as a whole is complete for the first time.
The apparatus illustrated in this example is designed so that importance is placed on ensuring the fixing capability for toner that is attained when performing consecutive printing of papers (approximately several hundreds of papers) that immediately follows warming-up and, to this end, the fixing rollers are increased in heat capacity and the accumulated amount of heat is thereby increased. For this reason, the warming-up length of time for the fixing unit is made greater than the added-up length of time that is needed for executing the preliminary rotating operations for the photosensitive body, etc. and the image correction operation. For this reason, even when preliminary rotating operations and image correction operation are executed, they are done by the point of time at which warming-up is complete. Namely, the warming-up length of time for the image forming apparatus as a whole falls within, and depends on, the warming-up length of time for the fixing heater. Therefore, the period of time until completion of the fixing warming-up operation is effectively utilized.
By applying the above-described various kinds of techniques and by somewhat victimizing ensuring the fixing capability in consecutively printing papers after warming-up, the fixing warming-up length of time is shortened. However, when the fixing warming-up length of time is shortened, as illustrated as an example in the timing chart of FIG. 3 the length of time that is taken for executing the preliminary rotating operations for the photosensitive body and developing unit and the image correction operation governs the warming-up length of time for the whole apparatus this time. For this reason, when the preliminary rotation for the photosensitive body and the image correction operation are executed for ensuring the appropriate quality of image, for example, at the earliest time of a day, in parallel with the fixing warming-up operation, whatever extent the fixing warming-up length of time is made short to, there was a problem that it was impossible to sufficiently shorten the warming-up length of time for the whole apparatus.